addset: split simple and ordered iteration

We have two goals here. First, we would like to restore the former iteration
order we had in 2.9. Second, we want this logic to be reusable for `fastasc`
and `fastdesc` methods.

mercurial/revset.py

@@ -2638,17 +2638,35 b' class _addset(_orderedsetmixin):'
         same time, yielding only one value at a time in the given order.
         """
         if not self._iter:
+            if self._ascending is None:
             def gen():
-                if self._ascending is None:
                     for r in self._r1:
                         yield r
                     s = self._r1.set()
                     for r in self._r2:
                         if r not in s:
                             yield r
+                gen = gen()
                 else:
                     iter1 = iter(self._r1)
                     iter2 = iter(self._r2)
+                gen = self._iterordered(self._ascending, iter1, iter2)
+            self._iter = _generatorset(gen)
+        return self._iter
+
+    def __iter__(self):
+        if self._genlist:
+            return iter(self._genlist)
+        return iter(self._iterator())
+
+    def _iterordered(self, ascending, iter1, iter2):
+        """produce an ordered iteration from two iterators with the same order
+
+        The ascending is used to indicated the iteration direction.
+        """
+        choice = max
+        if ascending:
+            choice = min
 
                     val1 = None
                     val2 = None
@@ -2682,15 +2700,6 b' class _addset(_orderedsetmixin):'
                         for val in it:
                             yield val
 
-            self._iter = _generatorset(gen())
-
-        return self._iter
-
-    def __iter__(self):
-        if self._genlist:
-            return iter(self._genlist)
-        return iter(self._iterator())
-
     def __contains__(self, x):
         return x in self._r1 or x in self._r2